1. Field of the Invention
This invention relates to a lighting apparatus, more particularly to a lighting apparatus capable of casting rectangularly-distributed illumination.
2. Description of the Related Art
A conventional lighting device on a street, such as a street lamp, usually casts circularly-distributed or elliptically-distributed illumination. When two street lamps are disposed in close proximity, regions of illumination thereof overlap, wherein the overlapping regions are overly bright when the extent of overlap is too large and are dim when the extent of overlap is too small. For vehicles that do not travel at high speeds, such differences in the intensity of illumination do not pose any considerable risk to drivers of the vehicles. However, when vehicles travel at high speeds on an Expressway, such differences in the intensity of illumination often pose unsafe driving conditions. Therefore, it has been proposed heretofore for the street lamps to project rectangularly-distributed illumination in order to improve road safety.
As illustrated in FIGS. 1 and 2, a conventional lighting device 1 that provides rectangularly-distributed illumination includes a reflective body 11, a light source 12, and a lens unit 13. The reflective body 11 is shaped in the form of a hollow four-sided diverging body, and has a rectangular opening 111, and four reflective surfaces 113 respectively extending from the four sides of the opening 111 and cooperating with each other to define a compartment 112. The light source 12 is disposed in the compartment 112 and is positioned centrally in the reflective body 11. A portion of the light emitted by the light source 12 is directed towards the opening 111. Another portion of the light emitted by the light source 12 is reflected by the reflective surfaces 113 so as to travel towards the opening 111.
With further reference to FIGS. 3 and 4, the lens unit 13 is disposed to close the opening 111 and permits passage of the light generated by the light source 12 therethrough. The lens unit 13 is a Fresnel lens that has a plurality of concentric annular triangular structures 131.
Although the lighting apparatus 1 is capable of providing rectangularly-distributed illumination, the rectangular distribution is achieved primarily through reflection of the light by the reflective surfaces 113. It is noted that a large portion of light energy is exhausted during the process of reflection, i.e., before the light exits the reflective body 11. In order to make up for the loss of the light energy and in order to comply with the standard illumination requirement, an increase of the power consumption by the light source 12 is needed, which is not economical.